Volume range control circuits



Oct. 1,1940.

H. PFISTER VOLUME RANGE CONTROL CIRCUITS Filed Feb. 5, 1938 Up b T0CONTROLLED TUBE D m .m 0 RT R E 5 T B TF m L m c w m m E B U H hATTORNEY.

Patented Oct. 1, 1940 UNITED STATES PATENT OFFICE VOLUlVIE RANGE CONTROLCIRCUITS tion of Germany Application February 5, 1938, Serial No.188,853 In Germany February 6, 1937 1 Claim.

The present invention relates to the production of a control potentialof definite characteristics for amplifiers with automatic volume rangecontrol; and particularly for obtaining a logarithmic 5 control curve inamplifiers in which the control action is efiected in a stage succeedingthe stage from which the control potential is tapped (forward control).A number of circuit schemes operating with current-dependent resistancesare known for securing a logarithmic control curve which will produce,due to the logarithmic power of discrimination of the human ear foracoustic intensity, the least perceptional impression of a change. Thesimplest arrangements use dry rectifiers having the requisitevoltage-current characteristics. Such rectifiers are arranged as voltagedividers for obtaining the desired control potential characteristic.However, circuit organizations of this kind possess the drawback thatthe characteristic of these rectifiers is markedly dependent on theambient temperature. It is, therefore, desirable to use, in this part ofthe circuit, tubes rather than current-dependent, temperature-affectedresistances. However, these tubes must not have any unduly low internalresistance; for this reason the commercial type of diodes cannot beemployed. In addition, diodes inside the normal operating range, possessa practically constant internal resistance. The use 30 of commercialtriodes, although their characteristic is unaffected by temperature, isnot recommendable for the reason that comparatively high platepotentials are required, and, also, because of the difiiculty ofrendering the resistance char- 35 acteristic sufficiently variable.

According to the invention, as current-dependent resistances tubescomprising a plurality of auxiliary electrodes, for instancespace-charge grid tubes, are used. It may be suitable to pro- 40 vide anadditional control at one of the auxiliary electrodes, for instance, atthe negatively biased grid of the space-charge grid tube.

In the drawing:

Figs. 1 and 2 illustrate two embodiments of the 5 invention.

Fig. 1 shows, by way of example, the basic circuit diagram for producinga logarithmic control voltage. As current-dependent resistance aspace-charge double grid tube R is chosen. The

50 tube direct current voltage, which simultaneously is the controlpotential for the stage to be regulated, is tapped through a condenser Cwhich is connected to a full-wave rectifier D rectifying part of thesignal input voltage. Resistor W is 55 shunted across the condenser C ina manner well known in the art. Resistor R1 is placed between thecathode of the full-wave rectifier D and the plate of v the space-chargegrid tube R. The space-charge grid G1 is impressed with a positivebiasing voltage, whereas the grid G2 is negatively biased. The controlpotential is fed to the controlled tube across the terminals K1 and K2.The particular way of connecting these terminals depends on the designof the controlled tube, and the particular purpose to be attained(expansion or compression of the dynamic range).

The control voltage UR is determined by the voltage U0 at the condenserC as Well as by the resistance R1 and the internal resistance R1 of thetube R. It is This equation must represent a definite function for adefinite required control characteristic within the control range. Theensuing characteristic of UR as a function of U0 is governed by thevoltage at the space-charged grid G1 and by the biasing potential at thecontrol grid G2. These voltages are preferably adjustable. Fig. 2 showsa circuit organization operatin with automatic control. In thisexemplified embodiment, the grid biasing voltage at the negativelybiased grid is a function of the direct current control potential. Thecircuit shown corresponds essentially to the circuit of Fig. 1. Only theresistance R1 is provided with a tap T1, connected by way of a voltagedivider R2 to the grid potential source. The grid bias potential of gridG2 is adjusted by means of the tap T2. In this arangement thecharacteristic of UR as a function of Us is also dependent on theadjustment of the automatic grid control as determined by the positionof the potentiometer tap at the resistance R1, as well as by theconnection of the control grid to the potentiometer tap at resistanceR2. If, during operation, fluctuations of emission of the tube R becomenoticeable, these may be compensated in a simple way by re-adjusting thetaps T1 and T2 at the resistances R1 and R2. The control grid isgenerally impressed with a negative biasing voltage, though in somespecial cases this may be dispensed with and then the ensuing gridcurrent will establish a further functional dependence of the controlpotential UR. In lieu of the space-charge grid type of tube as shown inthe embodiments here illustratively described, other types of multi-gridtubes possessing a suitable characteristic, such as hexodes, can, ofcourse, be used.

voltage connections between the cathode and plate, means applying anadjustable positive potential to the grid nearest the cathode wherebythe grid acts as a space charge grid and reduces the internal resistanceof the tube by an adjustable amount, means maintaining said platepositive, and means for applying to the grid between the plate and spacecharge grid a potential derived in part from a point on said resistorand in part from an adjustable source of negative potential.

HUBERT PFISTER.

